Low Birth Weight
A baby born smaller than the usual healthy range
Quick Facts
- Type: Newborn finding
- Usual threshold: Under about 2,500 g (5 lb 8 oz)
- Common causes: Prematurity, growth restriction
- Care setting: May need extra newborn or special care
Overview
Low birth weight describes a baby who is born weighing less than the usual healthy range, generally taken as under about 2,500 grams (5 pounds 8 ounces). It is a measurement rather than a diagnosis, and it tells doctors that a newborn may need closer attention in the early days and weeks of life.
A baby can have a low birth weight for two main reasons: being born early, before having time to grow fully, or growing more slowly than expected during pregnancy. Some babies are simply small but healthy. Babies with a low birth weight can be more vulnerable to certain difficulties around birth, but with appropriate care many do very well, and the outlook depends greatly on the cause and the baby's overall health.
Common Causes
Low birth weight usually results from one or both of the following:
- Being born early: Babies born before term have had less time to grow; see preterm birth.
- Slow growth in the womb: When a baby does not grow as expected during pregnancy, known as intrauterine growth restriction.
Contributing factors can include problems with the placenta, high blood pressure or other illness in the mother, multiple pregnancy (twins or more), smoking, alcohol or substance use in pregnancy, poor nutrition, and certain infections or genetic conditions. Some smaller babies are simply constitutionally small and entirely healthy.
Associated Symptoms
Babies with a low birth weight may show or be at risk of:
- Difficulty keeping warm, because they have less body fat
- Feeding difficulties and slow weight gain (failure to thrive)
- Low blood sugar in the early days
- Breathing difficulties, especially if born early
- Jaundice, a yellowing of the skin and eyes
- A greater susceptibility to infections
Diagnosis & Evaluation
Low birth weight is identified by weighing the baby at birth and comparing the weight with charts for the baby's gestational age. The cause is then explored.
- Weight and growth measurements: Comparing weight, length, and head size against standard charts.
- Assessment of gestational age: To distinguish prematurity from growth restriction.
- Pregnancy review: Looking at the placenta, the mother's health, and any risk factors.
- Newborn checks: Blood sugar, temperature, breathing, and feeding, with further tests if needed.
Treatment & Management
Care focuses on supporting the baby's growth, warmth, feeding, and health.
- Keeping warm: Skin-to-skin contact and, where needed, an incubator help maintain temperature.
- Feeding support: Frequent feeds, with breast milk encouraged; some babies need tube feeding or extra nutrition at first.
- Monitoring: Of blood sugar, breathing, and weight gain, sometimes in a special care or neonatal unit.
- Treating complications: Such as jaundice or infection if they arise.
- Follow-up: Ongoing checks of growth and development after going home.
Self-Care & Prevention
Not every cause of low birth weight can be prevented, but healthy pregnancy care lowers the risk and supports a baby's growth:
- Attend antenatal care: Regular check-ups allow growth and the placenta to be monitored, and problems to be managed early.
- Avoid smoking, alcohol, and recreational drugs: All are linked to smaller birth weight and other risks, so avoiding them is one of the most protective steps.
- Eat well: A balanced diet with recommended supplements, such as folic acid, supports healthy development.
- Manage existing conditions: Good control of conditions such as high blood pressure or diabetes in pregnancy helps the baby grow.
- Allow time between pregnancies: Spacing pregnancies can support better outcomes.
After birth, skin-to-skin contact, support with feeding, and keeping the baby warm all help a small baby thrive, and the care team will guide parents through these steps.
When to See a Doctor
Babies with a low birth weight are usually monitored closely by their care team from birth. After going home, seek prompt medical care if a baby:
- Is feeding poorly or not gaining weight
- Is unusually sleepy, floppy, or difficult to wake
- Has a fever, or feels cold and cannot be warmed
- Is breathing rapidly, grunting, or has a blue tinge to the lips
- Develops worsening jaundice, or has fewer wet nappies
Any baby who seems seriously unwell needs urgent assessment without delay.
Frequently Asked Questions
What is considered a low birth weight?
A low birth weight is generally a weight below about 2,500 grams (5 pounds 8 ounces) at birth. It is a measurement that signals a baby may need closer monitoring, and it can result from being born early, growing slowly in the womb, or simply being constitutionally small.
What causes a baby to be born with a low birth weight?
The main causes are being born early and slow growth in the womb. Contributing factors include placental problems, the mother's health, multiple pregnancy, smoking, alcohol or substance use, and poor nutrition. Some small babies are entirely healthy.
Are low birth weight babies healthy?
Many low birth weight babies do very well with appropriate care. They can be more vulnerable to difficulties such as trouble keeping warm, feeding problems, low blood sugar, jaundice, and infections, so they are monitored closely. The outlook depends on the cause and the baby's overall health.
When should I seek help for a small baby at home?
Seek prompt care if a baby feeds poorly, is not gaining weight, is unusually sleepy or floppy, has a fever or feels cold, is breathing rapidly or grunting, or has worsening jaundice. Any baby who seems seriously unwell needs urgent assessment.
References
- Centers for Disease Control and Prevention (CDC). Low Birthweight.
- MedlinePlus, U.S. National Library of Medicine. Low birth weight.
- World Health Organization (WHO). Low birth weight.